Patient monitoring systems are commonly used for monitoring the condition of a patient, such as in coronary care units and intensive care units of a hospital. Such systems typically include a bedside monitor having one or more sensors, such as ECG sensors, blood pressure sensors and temperature sensors, attached to the patient. The sensors measure various physiological parameters of the patient. The measured parameters are processed by a system processor and may be displayed on a video display screen and stored for later analysis. Patient physiological information from several bedside monitors may be forwarded to a central station located, for example, at a nursing station.
The bedside patient monitor and the central station may display physiological parameters as waveforms and/or numerical values. Another important function of patient monitoring systems is to generate alarms when one or more of the physiological parameters indicates that the patient requires attention. Such alarms are necessary because it is not feasible for the display screen of the patient monitoring system to be observed continuously. Alarms are typically annunciated both visibly and audibly.
The conventional way of specifying alarm criteria, shown in FIG. 5, is to set an upper threshold 102 and a lower threshold 104 for a measurement, such as heart rate. When the measured value goes above the upper threshold 102 or below the lower threshold 104, an alarm is issued. The thresholds are fixed as a function of time, except for exclusionary zones 106 and 108 which permit brief excursions outside the thresholds without issuing an alarm. It may be difficult to select thresholds which produce clinically acceptable results, while avoiding excessive false alarms. When the threshold is close to the desired value, false alarms may occur frequently; and when the threshold is spaced from the desired value, clinically significant alarm conditions may not be detected. In some cases, users of the patient monitoring system may wish to have the monitoring system generate an alarm when there is a sudden but significant change in the patient's condition or when there is a slow, gradual deterioration in the patient's condition. The conventional fixed thresholds do not adequately detect these conditions. Others have combined such thresholds with other techniques, such as fixed delays, hysteresis, or refractory periods. A problem with processes that use combinations of these techniques is that as additional constraints are applied to the process, the process complexity rises dramatically, and quickly reaches a point where implementing new functionality is not economically feasible; thus, it is desirable to be able to describe complex timing conditions and thresholds in a single process. Various, more sophisticated alarm criteria have been proposed. See, for example, J. H. Philip, "Thoughtful Alarms", in J. S. Gravenstein et al, eds. Essential Noninvasive Monitoring in Anesthesia,1980, page 191-201, and J. H. Philip, "Overview: Creating Practical Alarms for the Future", 1989. Such systems may be unnecessarily complex for relatively straightforward patient monitoring requirements.